Fuel injection into the cylinders of an internal combustion engine is most commonly acheived using either a unit injector system or a fuel distribution type system. In the unit injector system, fuel is pumped from a source by way of a low pressure rotary pump or gear pump to high pressure pumps, known as unit injectors, associated with corresponding engine cylinders for increasing the fuel pressure while providing a finely atomized fuel spray into the combustion chamber. Such unit injectors conventionally include a positive displacement plunger driven by a cam which is mounted on an engine driven camshaft. The fuel distribution type system, on the other hand, supplies high pressure fuel to injectors which do not pump the fuel but only direct and atomize the fuel spray into the combustion chamber.
A commonly used injector in both the unit injector and fuel distribution systems is a closed-nozzle injector. Closed-nozzle injectors include a nozzle assembly having a spring-biased tip valve element positioned adjacent the nozzle orifices for resisting blow back of exhaust gas into the pumping or metering chamber of the injector while allowing fuel to be injected into the cylinder. The tip valve element also functions to provide a deliberate, abrupt end to fuel injection thereby preventing a secondary injection which causes unburned hydrocarbons in the exhaust. For example, commonly owned U.S. Pat. No. 4,463,901 to Perr et al. disclosed a unit injector incorporating a conventional tip nozzle assembly including a valve element normally biased by a nozzle spring to block the nozzle orifices. When the pressure of the fuel within the nozzle cavity exceeds the biasing force of the nozzle spring, the tip valve element moves outwardly to allow fuel to pass through the nozzle orifices.
Recent and upcoming legislation resulting from a concern to improve fuel economy and reduce emissions continues to place strict emissions standards on engine manufacturers. In order for new engines to meet these standards, it is necessary to produce fuel injector systems capable of achieving higher injection pressures while maintaining accurate and reliable control of the metering and timing functions. As a result, closed-nozzle fuel injectors are undergoing structural modifications which better enable the injectors to produce and withstand the higher injection pressures. However, these improvements often undesirably increase the size by the injector which must conform to overall size restrictions dictated by the mounting arrangement on the engine. Therefore, fuel injector manufactures are continually seeking ways of maintaining or minimizing the overall size of the injectors while also incorporating the improvements. Moreover, any reduction in length or width of an injector often results in a decrease in the total size and weight of the engine thereby advancing the continuing effort to produce a compact, lightweight engine.
U.S. Pat. No. 4,531,672 to Smith (FIGS. 1 and 6) and U.S. patent application Ser. No. 065,583 (FIG. 2) entitled Individual Timing and Injection Fuel Metering System, both commonly assigned to the assignee of the present application, disclose unit fuel injectors having a conventional closed nozzle assembly. These closed nozzle injectors include outer and inner plungers positioned in an axial bore formed in the one or more barrels of the injector body to create a timing chamber between the plungers. A metering chamber is formed in the axial bore between the lower plunger and a spacer or disc which abuts the barrel to form the lower portion of the metering chamber. The closed nozzle assembly includes a nozzle housing having a nozzle cavity for housing a tip valve element and a spring housing for housing the biasing spring which biases the nozzle tip valve element in the closed position. The spring housing is positioned in compressive abutting relationship between the spring housing and the spacer. The bias spring, having a small diameter, is positioned in a bore centrally formed in the spring housing. As a result, a centrally disposed outer surface, provided in these embodiments by the spacer, must be positioned at the outer end of this central bore to support one end of the spring. Also, the metering chamber can not be positioned as far inward in the injector body as is desired to minimize the length of the injector. In addition, fuel transfer passages extending between the metering chamber and the nozzle cavity must be routed through the spacer around the spring housing central bore in the annular portion of the spring housing resulting in an unnecessarily large trapped fuel volume and consequently a longer than required response time. Moreover, the high pressure joint formed between the barrel and the spacer adjacent the metering chamber is extremely difficult to effectively seal.
As discussed above, many injectors incorporate features which, advantageously create and withstand higher injection pressures while undesirably increasing the overall size of the injector. For example, U.S. patent application Ser. No. 898,818, commonly assigned to the assignee of the present application, discloses a unit injector including three plungers which form an intensifier assembly as compared to the common diameter, two plunger injector of U.S. Pat. No. 4,463,901. The intensification assembly includes an upper and an intermediate plunger having larger diameters, and therefore larger cross-sectional areas, than the lower plunger. As a result, during each injection stroke of the plungers, an intensification effect caused by difference in cross-sectional areas of the plungers, increases the pressure of the fuel in the metering chamber and therefore the injection pressure. The larger diameter timing chamber also decreases the timing fluid pressure created in the timing chamber during the injection stroke thereby decreasing the stresses and forces in the outer portion of the injector body. However, this three-plunger design also undesirably increases the overall length and cost of the injector while increasing the size and weight of the engine.
U.S. Pat. Nos. 2,959,360 to Nichols and 3,379,374 to Mekkes disclose other types of closed nozzle assemblies for fuel injectors. Nichols discloses a closed nozzle assembly having a needle valve biased in a closed position by a coil spring. A fuel passage extends through the needle valve which is positioned at least partially within the coils of the spring. However, in order to control the needle valve and therefore injection, this assembly requires additional components, such as a movable housing and plunger assembly, positioned outward of the spring thereby undesirably increasing the length of the closed nozzle assembly and the injector. Moreover, the fuel passages extending through the needle valve do not supply fuel to the injector orifices for injection. As a result, longer and larger passages positioned outside the spring deliver a larger amount of fuel to the closed nozzle assembly than is necessary for injection resulting in a longer than necessary response time for each injection event.
U.S. Pat. No. 3,379,374 to Mekkes discloses a closed nozzle assembly having a spring-biased needle valve for controlling the flow of fuel from the injector orifices wherein the needle valve and fuel transfer passages are positioned within the spring coils. However, as a result, the needle valve is not positioned adjacent the injector orifices thus requiring an unnecessarily long transfer passage between the needle valve seat and the orifices. Consequently, after the needle valve closes, excess fuel left in this transfer passage may flow into the engine cylinder preventing the needle valve from effectively performing its intended function of providing a deliberate, abrupt end to injection, and disadvantageously causing secondary injections and unburned hydrocarbons in the exhaust. Also, by incorporating large, movable components within the spring coils, this closed nozzle assembly prevents other components, such as supply passages and a check valve, from being positioned within the spring coils.
Consequently, there is a need for a closed nozzle assembly which minimizes the length of the nozzle assembly and therefore the overall length of the injector while effectively functioning to control the discharge of fuel through the discharge orifices.